This disclosure relates to a downhole controlled-frequency seismic source that can output a relatively low-frequency oscillating seismic signal.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Seismic imaging may be used to identify characteristics or features of a geological formation. Among other things, these characteristics and features may include the presence or absence of certain lithological features, hydrocarbons, gases, and so forth. To obtain a seismic image, a seismic source may output a seismic signal into the geological formation. A seismic receiver may detect a resulting seismic signal that occurs after passing through the geological formation. Depending on the constituency of the geological formation, the receiver will detect certain variations in the seismic signal. Thus, the detected seismic signal may be used to determine certain properties of the geological formation, such as the lithology of the geological formation or the contents of a hydrocarbon zone in the geological formation.
Seismic sources may be deployed on the surface of the geological formation or in a borehole. Many seismic sources are impulsive, using explosives or airguns to emit a seismic signal into the geological formation. In cases where a seismic source is deployed downhole, the seismic source may be coupled to the geological formation—that is, connected to the geological formation so as to permit force from the seismic source to be transmitted into the geological formation—using a fluid. This is referred to as “fluid coupling.” Although fluid coupling may be effective for impulsive seismic sources, fluid may absorb low-frequency seismic energy. Thus, many downhole seismic sources may not be able to adequately provide low-frequency seismic signals.